MRI studies have found fit children have hippocampi about 12% larger than unfit children.
In his book Boost Your Brain, Johns Hopkins neurologist Majid Totuhi, M.D., Ph.D. shows MRI scans with 16% increases in the size of the hippocampus after a vigorous exercise regimen.
Arthur Kramer’s lab at U of I also found measurable increases in gray and white matter after 6 months of exercise in older adults. (S. J. Colcombe, K. I. Erickson, P. E. Scalf, J. S. Kim, R. Prakash, E. McAuley, S. Elavsky, D. X. Marquez, L. Hu and A. F. Kramer, "Aerobic Exercise Training Increases Brain Volume in Aging Humans," The Journals of Gerontology; Series A, vol. 61, no. 11, pp. 1166-1170, 2006).
Besides exercise, you can increase brain regions with study (Maguire, E. A.; Gadian, D. G.; Johnsrude, I. S.; Good, C. D.; Ashburner, J.; Frackowiak, R. S. J.; Frith, C. D., “Navigation-related structural change in the hippocampi of taxi Drivers,” Proceedings of the National Academy of Sciences. 97 (8): 4398–4403 (2000) and Katherine Woodlett and Eleanor Maguire, “Acquiring "the Knowledge" of London’s Layout Drives Structural Brain Changes,” Current Biology 21, 2109-2114 (2011).) and meditation (Britta K. Hölzel, James Carmody, Mark Vangel, Christina Congleton, Sita M. Yerramsetti, Tim Gard, and Sara W. Lazar, “Mindfulness Practice Leads to Increases in Regional Brain Gray Matter Density,” Psychiatry Research: Neuroimaging 191, no. 1 (2011): 36–43)